Speed control of DC shunt motor.

Experiment No.: – 02

Aim of The Experiment: –

Speed control of DC shunt motor.

Apparatus Required: –

Sl. No.NameSpecificationQuantity
01DC Shunt Motor2 BHP, 1440 rpm, 110V, 18.1A1 nos.
02Voltmeter(0-150)V, PMMC1 nos.
03Ammeter(0-10)A, PMMC1 nos.
04Ammeter(0-5)A, PMMC2 nos.
05MultimeterDigital Type2 nos.
06TachometerDigital Type1 nos.
07Connecting wirePVC Insulated copperas per required
08Rheostat(0-80)Ω2 nos.
Table No. 2.1

Circuit diagram: –

Theory: –

Any DC motor can be made to have smooth and effective control of speed over a wide range. The shunt motor runs at a speed defined by the expression:

\[\color\red{Eb\;= \;\dfrac{ZNɸP}{60*A}}\] \[\color\red{N\;=\; \dfrac{V-I_aR_a }{Kɸ}}\] \[\color\red{E_b\;=\; {V-I_aR_a}}\] \[\color\red{(K= \dfrac{PZ}{60A})}\] \[\color\red{Since\; I_aR_a\; drop\; is\; negligible \;N α V \;}\] \[\color\red{or\; E_b \;and\; N α \dfrac{1}{ɸ} \;or\; N α\; \dfrac{1}{I_a}}\]

Where N is the speed, V is applied Voltage, Ia is the armature current and Ra is the armature resistance and ɸ is the field flux.

Methods of speed control of DC Shunt Motor:

  1. Applied Voltage Control.
  2. Armature Voltage Control.
  3. Field flux/current Control.

Armature Voltage Control:

Speed control is achieved by adding an external resistance in the armature circuit. This method is used where fixed voltage is available. In this method a high current rating rheostat is required.

Disadvantages:

a. Large amount of power is lost as heat in the rheostat. Hence the efficiency is low.

b. Speed above the rated speed is not possible. In this method motor run below its rated speed.

Field Current Control Method:

Speed control by adjusting the air gap flux is achieved by means of adjusting the field current i.e by adding an external resistance in the field circuit.

The disadvantage of this method is that at low field flux the armature current will be high for the same load.

This method is used to run the motor above its rated speed only.

Procedures: –

for Armature Voltage Control:

  1. Connect all the apparatus as per circuit diagram.
  2. Keep the external resistance at minimum value in the armature and in the field circuit.
  3. Gradually increase the voltage till the motor attains the rated voltage.
  4. Increase the external resistance in the armature circuit and record the speed at various armature currents.

for Field Current Control Method:

  1. Connect the apparatus as per circuit diagram.
  2. Connect the external rheostat in armature circuit and field circuit of D.C Motor.
  3. Keep at maximum value of resistance in armature circuit and minimum value of resistance in the field circuit of the D.C motor.
  4. Gradually increase the voltage till the motor attains the rated voltage.
  5. Increase the external resistance in the field circuit and record the readings as per observation table.

Precaution:

  1. Check the polarity of the Supply voltage, all instruments as well as D.C motor terminals.
  2. Don’t forgot to increase the external resistance in the armature circuit at the time of starting of D.C motor.

Observation Table: –

Sl. No.Applied Voltage (VS) in volts.Field Current (If) in amps.Armature Voltage (VA) in volts.Armature Current (Ia) in amps.External Resistance (R2) in ohms.Speed (N in rpm)
01
02
03
04
05
06
07
08
Table No. 2.2

Plot the graph between armature current and speed.

Conclusion: –

To be written by student.

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